Ian Keith Ludlow

Application of neural networks to the inverse light scattering problem for spheres

A new approach suitable for solving inverse problems in multiangle light scattering is presented. The method takes advantage of multidimensional function approximation capability of radial basis function neural networks. An algorithm for training the networks is described in detail. It is shown that the radius and refractive index of homogeneous spheres can be recovered accurately and quickly, with maximum relative errors of the order of 10(-3) and mean errors as low as 10(-5). The influence of the angular range of available scattering data on the loss of information and inversion accuracy is investigated, and it is shown that more than two thirds of input data can be removed before substantial degradation of accuracy occurs.

An instrument for the classification of airborne particles on the basis of size, shape, and count frequency

Abstract A laser light scattering instrument for the study of airborne particulate material is described. The instrument examines individual particles (primarily in the size range 1–10 μm equivalent diameter) at rates of up to 10,000 s −1 . Each particle is ascribed an asymmetry factor, or asphericity factor, based upon the spatial intensity distribution of the scattered laser radiation. This factor ranges from 0 for perfect spheres to 100 for long fibres. Particles are also ascribed a size parameter based upon the total equivalent scattering from a perfect sphere. Data may be displayed graphically in terms of size vs count frequency, asymmetry factor vs count frequency, or as an axonometric plot of size vas asymmetry factor vs count frequency. The latter three-dimensional dimensional surface is a characteristic signature of the aerosol under examination and preliminary results indicate the possibility of employing this form of data analysis to monitor small changes in aerosol composition, such as, for example, the presence of low concentrations of spheres (droplets) or fibres. Results are presented for incompressible solid spheres, liquid droplets, and mineral fibres.

Laser diffractometer for single-particle scattering measurements

A laser diffractometer (polar nephelometer) constructed for single-particle studies is described. It takes advantage of an array of optical fibres and a scanning disc to allow the use of a single photomultiplier tube for the detection of scattered light. A background subtraction scheme makes the instrument especially valuable in cases where background scattering exceeds the scattering from the observed particle, for example for living cells in aqueous media. The system can be easily integrated with particle trapping equipment such as laser tweezers or an electrodynamic balance.

A scanning diffractometer for the rapid analysis of microparticles and biological cells

Ian K Ludlow, and Paul H Kaye, ‘A scanning diffractometer for the rapid analysis of microparticles and biological cells’, Journal of Colloid and Interface Science, Vol 69 (3): 571-589, May 1979. The final published version is available at doi: http://dx.doi.org/10.1016/0021-9797(79)90145-0 Copyright

Compact optical trapping microscope using a diode laser

Single beam gradient force optical trapping (laser tweezers) is increasing in importance as a technique for microparticle and cell manipulation and for the measurement of piconewton forces in aqueous media. An optical trap constructed using an optical microscope and a diode laser is described here. The system incorporates novel beam steering optics allowing the positioning of the trapping spot within the field of view of the microscope. Despite its full functionality the system is compact, portable, relatively inexpensive and easy to construct and operate. A new scheme for trapping force calibration is also reported and measurements compared to theoretical results. The trapping efficiency is found to be similar to that obtained when using other trapping systems.

The influence of the cortex on protoplast dehydration in bacterial spores studied with light scattering

Individual, dormant spores ofBacillus sphaericus were studied with laser diffractometry. Correlation coefficients were obtained for the water content and radius of the protoplast and the water content and thickness of the integument of the spores. The coefficients showed that the states of the protoplast and the integument were interrelated. The water contents of the protoplast and the integument were positively correlated (ρ=0.73), and spores with thinner integuments had more dehydrated protoplasts. The coefficients were compared with qualitative predictions based on current models of the mechanism responsible for protoplast dehydration. The results did not support models involving an expansion of the cortex as the cause of the dehydration, but were consistent with cortex contraction and the model in which the cortex maintained rather than produced the dehydrated state of the protoplast.

A high-speed scanning laser diffractometer

A high-speed laser diffractometer is described which employs a novel scanned array or optical light guides to record the angular intensity variation of light scattered radially by a particle throughout an arc of 180 degrees C in approximately 2.8 ms. The instruments has proved successful in the analysis of micrometre-sized fibres and individual microspheres in liquid flow. Furthermore, potential exists in the analysis of precipitation reaction kinetics and aerosol dynamics involving rapid particle growth and coagulation.